Code division multiple access (CDMA) systems such as those conforming to the TIA standard IS-95 are designed to operate synchronously, such that the transmissions of each base station and mobile portable unit are produced in accordance with a common timing reference clock. In current deployments, synchronization is achieved through the use of Global Positioning System (GPS) receivers at each base station. These receivers extract frequency and timing information from the GPS satellite navigation system and use the information to control the timing of the base station transmissions. While this is practical in some regions (such as the United States), some mobile network operators are adverse to the use of GPS for synchronization because control of the GPS system resides with the US Department of Defense. In addition, planning, installation and maintenance of the GPS receivers at each cell site is costly.
There is accordingly a desire among the operators for a CDMA system that may be operated asynchronously. Asynchronous operation is also desirable for new systems derived from the IS-95 standard or “third generation” CDMA mobile radio systems.
However, asynchronous operation entails signal coding problems that are not anticipated in the IS-95 standard. In the CDMA system, forward channel messages are transmitted in a common frequency band, with each message being modulated by two unique codes that distinguish the message from others in the channel. The first of the codes is a repeating pseudo-noise (PN) code having low autocorrelation that is applied to every each message broadcast by a base station. The PN code is 32,768 chips in length and is repeated every 26.67 ms, or approximately 75 times every two seconds. The PN code is shared in common by all base stations of the system, but each base station begins its modulation cycle at a unique starting point in the PN sequence, sometimes referred to as its “offset”. Since the system operates synchronously, the offsets between base stations is essentially static. Therefore the messages broadcast by a given base station can be recognized through correlation with the PN code having that base station's predetermined offset. A second code that modulates each message is a 64 chip code known as a Walsh code. Each message broadcast by a base station is encoded with an individual Walsh code selected from a set of mutually orthogonal Walsh codes. Therefore, a particular message from a base station can be recognized by correlating the PN-demodulated signal from that base station with the Walsh code of the message.
Operation of IS-95 CDMA systems depends on the static nature of the PN code offsets. If base stations are operated asynchronously, i.e. without a common timing reference, differences in oscillator rates will cause the relative PN offsets between base stations to drift over time. This leads to two problems that are not anticipated by the IS-95 standard. First, in standard systems the signals of base stations are initially recognized by their predetermined offsets, and so functions such as handover of a mobile handset from one base station to another are premised on the ability of the system to determine the identities of base stations detected by a handset by comparing the offset of the detected signal with a list of the predetermined system offsets. However, in an asynchronous system, offsets are continually changing, and so there is no static offset by which to identify base station signals.
Further, the drift of offsets in an asynchronous system inevitably leads to states of “collision”, during which adjoining base stations are modulating their signals with the same portion of the common PN code at essentially the same time. Under these circumstances, a receiver that is within range of both base stations and being served by either will demodulate both the signal from its serving base station and any signal from the other base station that is modulated using the same Walsh code that it is tracking. This results in a degradation or disruption of service for the receiver that continues until the signals have drifted apart to a state of non-collision.
If left uncorrected, collisions among signals from asynchronous base transceiver stations occur periodically, with the period between collisions being of the order of several hours. The IS-95 PN timing offsets are in steps of 26 milliseconds, and so collision is possible each time the timing of a base station drifts by this amount. Assuming that an unsynchronized base station employs an ovenized crystal oscillator with a stability of 10−8 or better (such accuracy is needed to maintain the radio frequency transmissions within the required limits), a base station will complete one full cycle of drift in approximately 1.5 days. It may be expected that, as there are typically a group of base stations in a region, with a number of overlapped coverage areas, there will be several PN collisions within each drift cycle.
Further explanation of CDMA mobile communications may be found in the books by Theodore S. Rappaport: “Wireless Communications Principles and Practice”, IEEE Press 1996, ISBN 0-7807-1167-1 (Chapter 10 pages 519-533) and by Gerry D. Gibson: “The Mobile Communications Handbook”, IEEE Press 1996, ISBN 0-8493-8573-3 (Chapter 27 pages 430-448). The IS-95 CDMA standard may be found in the publication by the Telecommunications Industry Association: “Mobile Station-BaseStation Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System”, 1995 TIA/EIA/IS-95-A.